The present invention relates to apparatus for deflecting a pointer in proportion to the rotational speed of a shaft, and more particularly to tachometers, speedometers and the like of high accuracy.
Apparatus for deflecting a pointer in proportion to rotational speed of a shaft is found in many common devices such as tachometers, speedometers and the like. Most such apparatus operate on a governor principle. In such systems a governor spindle is connected to the rotating shaft for rotation therewith. A plurality of weights spaced around and from the circumference of the governor spindle are operably connected at one end to the governor spindle by means of a first spider, the hub of the first spider being mounted at a fixed point along the length of the governor spindle for rotation therewith, so that the weights rotate at the same speed as the governor spindle. The weights are connected at the other end thereof to a second spider which is also mounted on the governor spindle, but the second spider is displaceable axially along the length of the governor spindle. A governor spring acting along the governor spindle axis biases the first and second spiders apart.
Rotation of the shaft whose rotational speed is to be measured is transmitted to the first spider and thence to the weights. The rotation of the weights about the governor spindle creates a centrifugal force which causes the weights to move outwardly from the governor spindle. The centrifugal force acting on each weight is balanced by the governor spring which tends to maintain the weights close to the governor spindle. As the rotational speed of the shaft increases, the centrifugal force acting on the weight grows larger (F = Mrw.sup.2) and the weights move outwardly from the governor spindle. The outward movement of the weights draws the first and second spiders together by causing the second spider to slide axially along the governor spindle towards the first spider, thereby compressing the governor spring. The axial motion of the second spider ceases when the governor spring force causing separation of the spiders is equal to the axial component of the centrifugal force tending to draw the spiders together. Thus, the governor translates the rotational speed of the shaft into an axial movement along the governor spindle axis which is generally proportional to the rotational speed of the shaft. The axial movement is then translated into a pivotal movement of a pointer against a stationary dial, for example, by a rack and pinion arrangement.
It is generally desirable to have equal unit increments of the quantity to be measured (e.g. rpm, mph, etc.) equidistantly spaced around the circumference of the dial against which pointer movement is to be measured for accuracy. This means that the pointer movement must be directly proportional to the rotational speed of the shaft. However, as the centrifugal force acting on the weights varies as the square of the rotational speed of the shaft, the translational motion of the second spider, and hence the pointer motion, is not linearly proportional to the rotational speed of the shaft. Thus tachometers of the construction described above provide readings with a built-in error factor, the error factor generally increasing with the rotational speed of the shaft.
Accordingly, it is an object of the present invention to provide a tachometer or similar device in which pointer movement is linearly proportional to the rotational speed of the shaft.
It is also an object to provide such a device containing means to compensate for the lack of linear proportionality between the translational movement of the second spider and the rotational speed of the shaft.
It is a further object to provide such a device which is easily adaptable for measuring a broad range of rotational speeds.
Another object is to provide such a device which is of simple, rugged and inexpensive construction.